OLEDs which comprise an organic thin film layer which includes a light emitting layer located between an anode and a cathode are known in the art. In such devices, emission of light may be obtained from exciton energy, produced by recombination of a hole injected into a light emitting layer with an electron.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. Generally, OLEDs are comprised of several organic layers in which at least one of the layers can be made to electroluminesce by applying a voltage across the device. When a voltage is applied across a device, the cathode effectively reduces the adjacent organic layers (i.e., injects electrons), and the anode effectively oxidizes the adjacent organic layers (i.e., injects holes). Holes and electrons migrate across the device toward their respective oppositely charged electrodes. When a hole and electron meet on the same molecule, recombination is said to occur, and an exciton is formed. Recombination of the hole and electron in luminescent compounds is accompanied by radiative emission, thereby producing electroluminescence.
Depending on the spin states of the hole and electron, the exciton resulting from hole and electron recombination can have either a triplet or singlet spin state. Luminescence from a singlet exciton results in fluorescence, whereas luminescence from a triplet exciton results in phosphorescence. Statistically, for organic materials typically used in OLEDs, one quarter of the excitons are singlets, and the remaining three-quarters are triplets. Electro-phosphorescent OLEDs have been shown to have superior overall device efficiencies as compared with electro-fluorescent OLEDs.
Due to strong spin-orbit coupling that leads to singlet-triplet state mixing, heavy metal complexes often display efficient phosphorescent emission from such triplets at room temperature. Accordingly, OLEDs comprising such complexes have been shown to have internal quantum efficiencies of more than 75% (Adachi, et al., Appl. Phys. Lett., 2000, 77, 904). Certain organometallic iridium complexes have been reported as having intense phosphorescence (Lamansky, et al., Inorganic Chemistry, 2001, 40, 1704), and efficient OLEDs emitting in the green to red spectrum have been prepared with these complexes (Lamansky, et al., J. Am. Chem. Soc., 2001, 123, 4304). OLEDs, as described above, generally provide excellent luminous efficiency, image quality, power consumption and the ability to be incorporated into thin design products such as flat screens, and therefore hold many advantages over prior technology, such as cathode ray devices.
Development of light emitting phosphorescent materials in which light emission is obtained from a triplet exciton resulting in enhanced internal quantum efficiency have lead to OLEDs having greater current efficiency. Such phosphorescent materials may be used as a dopant in a host material which comprises such a light emitting layer.
In a light emitting layer formed by doping with a light emitting material such as a phosphorescent material, excitons can efficiently be produced from a charge injected into a host material. Exciton energy of an exciton produced may be transferred to a dopant, and light emission may be obtained from the dopant at high efficiency. Exitons may be formed either on the host materials or directly on the dopant.
In order to achieve intermolecular energy transfer from a host material to a phosphorescent dopant with high device efficiencies, the excited triplet energy Eg(T) of the host material must be greater than the Eg(T) of the phosphorescent dopant. In order to carry out intermolecular energy transfer from a host material to a phosphorescent dopant, the Eg(T) of the host material has to be larger than the Eg(T) of the phosphorescent dopant.
PCT Publication No. WO2011/048821 published on Apr. 28, 2011 discloses certain prior art OLED devices, Example 6 and Example 7, in which Ir(ppy)3 is provided as the electroluminescent dopant and a carbazole type compound is provided as an electron blocking layer between the electro luminescent layer and a hole injection layer. WO2011/048821 also discloses additional prior art OLED devices, Comparative Example 1 and Comparative Example 2, in which the electron blocking layer is omitted for comparison purposes. Table 1 provides the device performance data for the prior art devices Example 6, Example 7, Comparative Example 1 and Comparative Example 2.
TABLE 1At 10 mA/cm2PowerAt 1000 nits1931 CIEVoltageLuminanceEfficiencyVoltageLEEQEDevice #xy(V)(cd/m2)(lm/W)(V)(cd/A)(%)Example 6 from0.3090.6155.77260414.214.8729.268.40WO2011/048821Example 7 from0.3060.6205.68294316.304.7033.609.60WO2011/048821Comparative0.3160.6126.5419319.295.8920.745.94Example 1 fromWO2011/048821Comparative0.3120.6135.89189610.235.2820.235.76Example 2 fromWO2011/048821
Despite the recent discoveries of efficient heavy metal phosphors and the resulting advancements in OLED technology, there remains a need for longer device stability. Fabrication of devices that have longer lifetimes will contribute to the development of new OLED based display technologies and help realize the current goals toward full color electronic display on flat surfaces. An improved OLED device that exhibit improved lifetime is disclosed herein along with the host materials, phosphorescent emitter materials and hole transport materials that may be used to construct such OLED.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. As used herein, “small molecule” refers to any organic material that is not a polymer, i.e., organic material having molecules with a defined molecular weight, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances, e.g. oligomers. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.
As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.
As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.
A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.
As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.
As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.